Steam-cracking or pyrolysis of hydrocarbons is one of the core processes in the petrochemical industry [S. Raseev; “Thermal and Catalytic Processes in Petroleum Refining”, Marcel Dekker Inc., New York (2003), p137-274]. Current world production of steam-cracking products is estimated to be more than 100 million metric tons/year of ethylene and propylene.
Basically, steam-cracking comprises a step in which the hydrocarbon mixture to be transformed is mixed with steam and submitted to elevated temperatures in a tubular reactor. The reaction temperature usually ranges from about 700 to about 1000° C. according to the type of feedstock treated (the longer the hydrocarbon molecular structure, the lower the required temperature for cracking), while the residence time ranges from a fraction of second to a few seconds. The different resulting products, gaseous or liquid, are then collected and separated. The product distribution depends on both the nature of the initial hydrocarbon mixture and the reaction conditions.
During steam-cracking, light paraffins such as ethane, propane and butane (obtained mainly by extraction from various natural gas sources), as well as naphthas and other heavier petroleum cuts, are broken down (cracked) into mainly:
1) light olefins, primarily ethylene and propylene;
2) depending on the feedstock employed, a C4 cut rich in butadienes and a C5+ cut with a high content of aromatics, particularly benzene; and
3) hydrogen.
Since enormous quantities of hydrocarbons are steam-cracked throughout the world, even modest yield or product selectivity improvements may lead to substantial commercial advantages.
A method for upgrading the products of propane steam-cracking has been previously developed [R. Le Van Mao, U.S. Pat. No. 4,732,881]. This process comprises adding a small catalytic reactor to a conventional propane steam-cracker. The catalysts used were based on hybrid zeolite catalysts, namely a ZSM-5 zeolite modified with Al and Cr. Significant increases in the yield of ethylene and aromatics were obtained.
A further refined steam-cracking process has been previously described [R. Le Van Mao, S. Melancon, C. Gauthier-Campbell and P. Kletnieks; Catal. Lett. 73 (2/4) (2001) 181; R. Le Van Mao, PCT/CA00/01327] (WO0132806). The process comprises the use of a tubular reactor with two heating zones positioned at the two ends of the reactor (referred to as a “dual” reactor). The first heating zone (I) is empty or contains a robust solid material which acts primarily as a heat transfer medium. The second heating zone (II) is charged with a ZSM-5 zeolite based catalyst, preferably of the hybrid configuration (i.e. at least two co-catalysts are commingled). Variations in the temperature of heating zone I versus heating zone II, as well as the textural properties and/or the surface composition of the catalyst of zone (II), were used to increase the conversion and to vary the product distribution, namely the ethylene/propylene ratio.
New steam cracking-catalysts requiring a simpler reactor technology (i.e. a single heating zone reactor or a “mono reactor”) have also been disclosed [R. Le Van Mao, PCT/CA01/01107 (WO 02/10313); S. Melancon, R. Le Van Mao, P. Klenieks, D. Ohayon, S. Intem, M. A. Saberi, and D. McCann, Catal. Lett. 80 (3/4), (2002), 103]. More specifically, mono-component and hybrid catalyst compositions for use in the cracking of hydrocarbon feeds are disclosed. The catalyst compositions comprise oxides of aluminum, silicon, chromium, and optionally, oxides of monovalent alkaline metals.
New and improved versions of catalysts for the selective deep catalytic cracking process comprising molybdenum or tungsten oxides and cerium oxide, supported on zirconium oxide, were described in International Application PCT/CA03/00105 (WO03064039).
There thus remains a need to develop selective and thermally stable cracking catalysts that can be used in both mono- and dual reactor configurations.
The present invention seeks to meet these and other needs.